The present invention is drawn to coatings for ink jet ink prints to improve the light fastness of a printed image and provide a water-resistant protective coating.
In recent years, computer printer technology has evolved to a point where very high resolution images can be transferred to various types of media, including paper. One particular type of printing involves the placement of small drops of a fluid ink onto a media surface in response to a digital signal. Typically, the fluid ink is placed or jetted onto the surface without physical contact between the printing device and the surface. Within this general technique, the specific method that the ink jet ink is deposited onto the printing surface varies from system to system, and can include continuous ink deposit and drop-on-demand ink deposit.
With regard to continuous printing systems, inks used are typically solvent based using solvents such as methyl ethyl ketone and ethanol. Essentially, continuous printing systems function as a stream of ink droplets are ejected and directed by a printer nozzle. The ink droplets are directed additionally with the assistance of an electrostatic charging device in close proximity to the nozzle. If the ink is not used on the desired printing surface, the ink is recycled for later use. With regard to drop-on-demand printing systems, the ink jet inks are typically aqueous based using water and/or glycols as solvents. Essentially, with these systems, ink droplets are propelled from a nozzle by heat or by a pressure wave such that all of the ink droplets ejected are used to form the printed image.
There are several reasons that ink jet printing has become a popular way of recording images on various media surfaces, particularly paper. Some of these reasons include low printer noise, capability of high speed recording, and multi-color recording. Additionally, these advantages can be obtained at a relatively low price to consumers. However, though there has been great improvement in ink jet printing, accompanying this improvement are increased demands by consumers in this area, e.g., higher speeds, higher resolution, full color image formation, increased stability, etc. As new ink jet inks are developed, there have been several traditional characteristics to consider when evaluating the ink in conjunction with a printing surface or substrate. Such characteristics include edge acuity and optical density of the image on the surface, dry time of the ink on the substrate, adhesion to the substrate, lack of deviation of ink droplets, presence of all dots, resistance of the ink after drying to water and other solvents, long term storage stability, and long term reliability without corrosion or nozzle clogging. Though the above list of characteristics provides a worthy goal to achieve, there are difficulties associated with satisfying all of the above characteristics. Often, the inclusion of an ink component meant to satisfy one of the above characteristics can prevent another characteristic from being met. Thus, most commercial inks for use in ink jet printers represent a compromise in an attempt to achieve at least an adequate response in meeting some or all of the above listed requirements.
In general, ink jet inks are either dye- or pigment-based inks. Dye-based ink jet inks generally use a soluble liquid colorant that is usually water-based to turn the media a specific color. Because of their makeup, dye-based inks are usually not waterproof and tend to be more affected by UV light. This results in the color changing over time, or fading. For optimum performance, this type of ink has often required that the proper media be selected in accordance with the application, thus, reducing the choice of media for printing. Conversely, pigmented inks typically use a particulate solid colorant to achieve color. In many cases, the line quality and accuracy of plots produced by pigment-based inks are usually superior to that of dye-based inks. With pigmented inks, solid particles adhere to the surface of the substrate. Once the water in the solution has evaporated, the particles will generally not go back into solution, and are therefore more waterproof. In addition, pigmented inks are much more UV resistant than dye-based inks, meaning that it takes much longer for noticeable fading to occur. Though pigmented inks, in some areas, exhibit superior characteristics, dyes tend to run cleaner, provide better yield, offer better particle size, and are easier to filter. Thus, dye-based inks have been more often used for common applications and have tended to be more chromatic and provide more highly saturated colors.
In order for ink jet prints to effectively compete with silver halide photography, one important improvement that must occur is that ink jet inks must improve their ability to remain stable to light exposure for longer periods of time. At this point in time, photographs typically will last much longer under prolonged light exposure, i.e., about 14-18 years under fluorescent light exposure. Conversely, some of the best ink jet printers will produce prints that last for only about 6-8 years under similar conditions. Particularly, with respect to dye-based ink jet ink, the phenomenon of discoloration occurs even more readily than is typical for pigment-based ink jet inks. However, as described above, dye-based inks are sometimes preferred because they are very convenient to use and have good distinction of color.
In the photographic industry, technologies have been developed which have been reported to last much longer than the typical 14-18 years, and even up to 60 years. However, ink jet print quality has improved steadily in recent years. Under the right circumstances, ink jet prints can look as bright and sharp as silver-halide photos. Thus, it is conceivable that ink jet printing may evolve to be the printing method of choice in the not too distant future. On the other hand, with many current technologies, ink jet prints can smear if they become wet or are overly handled. To stop the smearing, an overcoat can be placed over the print. Using toner to protect an image has been considered previously. For example, U.S. Pat. No. 5,847,738 describes the use of clear toner to overcoat ink jet prints. Additionally, in U.S. Pat. No. 5,804,341, a clear toner overcoat is disclosed for use with silver halide photos. These provide protection against abrasion. Additionally, U.S. Pat. No. 5,612,777 teaches the use of a clear overcoat containing UV absorbers to protect electrophotographic prints.
Additives have also been added to inks as well as coated on paper (prior to printing) to improve lightfastness once the ink is printed on the paper. For example, in U.S. Pat. No. 6,056,812, the entire teachings of which are incorporated herein by reference, an ink additive is disclosed to improve lightfastness and durability of the properties of various inks. As stated, either the additives are added to the ink itself, or the additive is used in a composition that is placed on a substrate prior to printing on the substrate. Thus, once the ink jet ink is printed on the coated substrate, e.g., paper, properties are imparted to the ink that promote lightfastness and durability.
However, none of the prior art provides a toner based coating that is specifically formulated to improve the light fastness of ink jet inks after printing of the characters or image on the substrate. Such a toner based coating, formulated specifically for application after printing can provide increased light fastness as well as provide the added benefit of protecting the prints from water or other damage due to exposure to the elements.
The present invention is drawn to a colorless toner composition for overcoating a print having an ink jet ink image printed thereon. The colorless toner comprises a toner resin, and at least two additives selected from the group consisting of ultraviolet absorbers, free radical inhibitors, thermal stabilizers, and combinations thereof. The toner resin to additive weight ratio can be from about 1:1 to 99:1 by weight. In a preferred embodiment, one of the at least two additives can be an ultraviolet absorber and the second additive can be selected from the group consisting of free radical inhibitors and thermal stabilizers.